Glasses with a high refractive index and their use as segments of multifocal corrective lenses.
The present invention pertains to optical glasses with a high refractive index, which can be used particularly for the manufacture of segments used in the manufacture of multifocal corrective lenses. These multifocal lenses comprise a principal glass lens and one or more glass segments which are fused to it and whose refractive indices, which are higher than that of the principal lens, make it possible to obtain zones with higher corrective power. Such segments are normally fused to tie ophthalmic lens with a thermal fusing treatment bringing the whole constituted by the, principal lens and the segment to a temperature near the softening point of the two glasses which make it up. In this manufacturing method, the glass in the segment must have characteristics of dilatation and viscosity in the vicinity of the softening temperature which are close to those of the glass forming the principal lens.
Glasses with a high refractive index which are appropriate for the production of segments of multifocal lenses have already been described in the past. One example is U.S. Pat. No. 4,351,906, which describes the glasses whose weight composition includes 28-32.5% SiO.sub.2, 28.5-34% PbO, 0-1% Li.sub.2 O, 2-9% NaO, 0-4% K.sub.2 O, 0-22% BaO, 0-20% La.sub.2 O.sub.3, 0-4% ZnO, 2-6.5% ZrO.sub.2, 0-9% TiO.sub.2, and 0-10% Nb.sub.2 O.sub.5 with, as supplementary conditions, 4%&lt;Li.sub.2 O+Na.sub.2 O+K.sub.2 O&lt;10.5%, 14%&lt;BaO+La.sub.2 O.sub.3 &lt;22% and 6%&lt;ZrO.sub.2 +TiO.sub.2 +Nb.sub.2 O.sub.5 &lt;17%. The corresponding glasses have a dilatation coefficient (in the range of 25-300.degree. C.) between 90 and 98.times.10.sup.-7 /.degree.C., a softening point between 640 and 700.degree. C., and a refractive index between 1.745 and 1.771. The disadvantage of these glasses is that they have high dilatation values and a high softening point.
U.S. Pat. No. 4,211,569 also describes glasses which are compatible with the manufacture of segments for ophthalmic lenses. Their characteristics are a dilatation coefficient (in the range of 25-300.degree. C.) between 60 and 66.times.10.sup.-7 /.degree.C., a softening point between 620 and 645.degree. C., and a refractive index between 1.58 and 1.71. Their composition is in the following range:
30-50% SiO.sub.2, 4-8% B.sub.2 O.sub.3, 25-50% PbO, 0-2% Li.sub.2 O, 0-3% Na.sub.2 O, 4-9% BaO, 0-3% La.sub.2 O.sub.3, 0-2% ZrO.sub.2, and 0.5-3% TiO.sub.2 with, as an additional condition:
1%&lt;Li.sub.2 O+Na.sub.2 O+K.sub.2 O&lt;4%.
The disadvantage of these glasses is that they have an insufficient refractive index for certain applications.
FR-A-2,550,524 describes glasses whose compositions by weight are within the following ranges:
23-32% SiO.sub.2, 4-7% B.sub.2 O.sub.3, 0.5-3% Al.sub.2 O.sub.3, 23-30% BaO, 15-30% PbO, 0.5-10% TiO.sub.2, 5-20% La.sub.2 O.sub.3, and 0-1% Nb.sub.2 O.sub.5. Their properties include a refractive index higher than 1.730, an Abbe coefficient of at least 33, a density not exceeding 4.45 g/cm.sup.3, and a thermal extrusion coefficient between 77 and 83.times.10.sup.-7 /.degree.C. The disadvantage of these glasses is that they have too high a dilatation coefficient for some applications.
More recently, U.S. Pat. No. 5,162,826 described glasses having the following compositions by weight:
24-42% SiO.sub.2, 3-7.5% Al.sub.2 O.sub.3, 2-7.5% B.sub.2 O.sub.3, 35-57% PbO, 0-2% Li.sub.2 O, 0-2% Na.sub.2 O, 0-3% K.sub.2 O, 0-8% La.sub.2 O.sub.3, 0-2.5% ZrO.sub.2, and 0-8% TiO.sub.2, with conditions such that the sum Li.sub.2 O+Na.sub.2 O+K.sub.2 O is less than 3.5% and that of La.sub.2 O.sub.3 +ZrO.sub.2 +TiO.sub.2 is between 4 and 12%. These glasses are devoid of alkaline-earth oxides, and have a thermal extrusion coefficient slightly too low for some applications.
Therefore, there is a need to have available glasses with a group of improved properties appropriate for certain applications, such as the manufacture of multifocal photochromic lenses.